Conventionally, in weft knitting machines, a plurality of knitting needles are arranged side by side on needle beds, and a knitting fabric is knitted by feeding knitting yarns while a knitting operation is performed sequentially with the knitting needles. The sequential knitting operation of the knitting needles is performed by knitting cam mechanisms that are mounted on carriages moving along the needle beds. Carriers that are brought by the carriages feed knitting yarns to the knitting needles. The knitting yarns are fed to the carriers from a constant position along the needle beds such as the end portion or the central portion. While a knitting fabric is knitted, the relationship between the position of the carriages that bring the carriers and the position at which knitting yarns are fed continuously changes. Knitting yarns are knitted into the knitting fabric and consumed on the whole. However, when the carrier moves closer to the position at which the knitting yarn is fed, outside the knitting fabric, the feeding path of the knitting yarn to the carrier is shortened, but the knitting yarn of an amount by which the feeding path is shortened is not knitted into the knitting fabric, and thus the tensile force of the knitting yarn is reduced, so that the knitting yarn comes loose.
Generally, after knitting is performed up to an end of a knitting fabric of one course in movement from the position at which a knitting yarn is fed to the carrier to a side away from the position, the carriage continuously moves some distance, and is then halted. In order to knit the knitting fabric for the next one course, the movement direction of the carriage is reversed. When the carriage is reversed, starts to move in the opposite direction, and brings the carrier, the knitting yarn is not consumed until the carriage passes by the end portion of the knitting fabric and resumes the knitting of the knitting fabric. Even, when the distance from the position at which the knitting yarn is fed to the carrier to the carrier decreases and the knitting yarn of an amount by which the distance decreases is not necessary, the knitting yarn of the amount by which the feeding path is shortened cannot be knitted into the knitting fabric, and thus the tensile force of the knitting yarn is reduced, so that the knitting yarn comes loose. When a knitting fabric is knitted, it is required that a fluctuation in the tensile force of a knitting yarn is small. The tensile force of a knitting yarn affects the size of knitting loops of a knitting fabric that is to be knitted, and thus the fluctuation in the tensile force makes the size of knitting loops of the knitting fabric non-uniform.
In manual knitting machines in which the carriage is manually moved, an operation is performed using actuation in tension of a spring material (see Japanese Unexamined Patent Publication JP-A 57-191352 (1982), for example). In weft knitting machines of an automated control type in which a knitting fabric is knitted based on knitting data while driving the carriage, other methods also can be applied. A braking device and pulling back means are provided on a feeding path of a knitting yarn, and based on a start signal that is generated before the carriage actually starts to move, the braking device binds a knitting yarn and the pulling back means in which the torque is increased pulls back the knitting yarn. A technique for preventing the tensile force of a knitting yarn from being reduced in this manner by controlling the torque in every reversing movement of the carriage has been proposed (see Japanese Unexamined Patent Publication JP-A 04-257352 (1992), for example).
It should be noted that when the carriage moves in a direction away from the position at which a knitting yarn is fed to the carrier, the distance to the carrier increases as the carriage moves. Especially when the position at which a knitting yarn is fed to the carrier is provided on an end portion on one side on the needle bed, and the carriage is reversed from that end portion to the end portion on the other side, the demand for the knitting yarn drastically increases. The applicant of the invention has proposed a technique for suppressing a fluctuation in the tensile force of a knitting yarn by predicting the demand for a knitting yarn and by performing PID control on a servomotor for driving rollers by which a knitting yarn is held and sent out (see Japanese Unexamined Patent Publication JP-A 2002-227064, for example).
Simply by using tension of a spring material as in JP-A 57-191352, it is impossible to sufficiently perform the slack reduction that is necessary in order to knit a knitting fabric of a high quality by suppressing a fluctuation in the tensile force of a knitting yarn in a weft knitting machine in which the productivity is increased by moving the carriage at a high speed. In JP-A 04-257352, it is intended to reduce the slack by controlling the torque. In this case, when the tensile force of a knitting yarn increases because the knitting yarn is hooked at a midpoint on a feeding path, for example, it is impossible to pull in the knitting yarn any more.
When a knitting yarn is held and sent out by rollers as in JP-A 2002-227064, it is possible to cope with the drastic demand for a knitting yarn, but when the knitting yarn comes loose due to the change in the position of the carrier, the slack cannot be absorbed as appropriate. Thus, it is required that a knitting yarn does not come loose even when the demand for the knitting yarn vanishes.